During a process of manufacturing a semiconductor, there has hitherto been used a photomask for use in transferring a circuit pattern onto a semiconductor wafer through exposure. A photomask of this type is usually used for exposure while being held such that a mask surface of the photomask is made substantially horizontal (or such that the normal line of the mask surface is oriented essentially in the direction of gravity). In order to enable accurate transfer, consideration must be given to preventing the mask surface from being bent under the influence of gravity or the like. With this being the case, various proposals have hitherto been put forth in connection with a holding method for retaining planarity of the photomask during exposure (see JP-A-9-306832 and JP-A-2000-223414).
When optical performance of the photomask, such as profile irregularity or a distribution of refractivity, is measured, consideration is given to holding a photomask in an upright position so as to minimize influence of gravity (i.e., in a position where the normal line of the photomask surface becomes essentially perpendicular to the direction of gravity). Minimizing the load, which must be exerted on said photomask for maintaining the above-arranged photomask still, to thus prevent occurrence of distortions in the sample is desirable.
However, even when the photomask is held in an arbitrary position, there still exists a possibility of occurrence of distortions, such as deflections, attributable to the held state of the photomask, so long as gravity or the load used for holding the photomask acts on the photomask.
From a technical standpoint, complete elimination of distortions attributable to such a held state is extremely difficult. Moreover, a technique for measuring such distortions while the distortions are isolated from the surface shape of the photomask has hitherto been unknown Therefore, in a related-art measurement technique, distortions attributable to a held state are unavoidably superimposed on the measurement result of a surface shape of the photomask. Extreme difficulty is encountered in high-precision measurement of a surface shape in such a distortion-free state.